The present invention relates to an optical printer apparatus capable of relatively moving on a sensitized sheet to expose it with given timing, thereby forming an image, and more specifically, to a technique for controlling the exposure timing of the optical printer apparatus.
Disclosed in Japanese Patent Application Laid-open No. 2-169270 is an optical printer apparatus in which an optical head is relatively moved on a sensitized sheet to form an image on the sensitized sheet. This optical printer apparatus will now be described with reference to FIG. 16.
A sensitized sheet 60 is driven at constant speed in the direction of arrow Z with respect to the optical head 10 by means of feed rollers 70. The optical head 10 comprises a white light source 20 for radially emitting white light, a cylindrical lens 30 for linearly converging the white light on the sensitized sheet 60, a three-color separation liquid crystal shutter 40, and a liquid crystal shutter 50.
The three-color separation liquid crystal shutter 40 is composed of three shutters 40r, 40g and 40b that linearly extend in the width direction (spreading direction) of the white light from the cylindrical lens 30. These three shutters 40r, 40g and 40b are driven independently of one another, and are provided individually with color filters that transmit red (R), green (G), and blue (B) light beams, respectively.
The liquid crystal shutter 50 includes a plurality of pixels that are arranged in the same direction as the lengthwise direction of the shutters 40r, 40g and 40b. 
The following is a description of a method for forming an image on the sensitized sheet 60 by means of the apparatus shown in FIG. 16.
The optical printer apparatus receives gradated color image data, controls the shutters 40r, 40g and 40b in accordance with the image data, and exposes the surface of the sensitized sheet 60, thereby forming the image thereon. After the shutter 40r opened, the shutter 40g opens for a predetermined time, and after the shutter 40g opened, the shutter 40b opens for a predetermined time, to transmit the white light. This predetermined time is just equal to a period of time during which the sensitized sheet 60 moves for a distance X in FIG. 16.
Thus, the sensitized sheet 60 is exposed to the red light beam (R), which is first transmitted through the shutter 40r, for the distance X in its moving direction (direction Z). Then, the shutter 40r is closed, while the shutter 40g opens. Since the sensitized sheet 60 is moved for the distance X by this time, that portion of the sensitized sheet 60 which has already been exposed to the light beam R is exposed again to the green light beam (G) that is transmitted through the shutter 40g. When the sensitized sheet 60 further moves for the distance X, thereafter, the portion already exposed to the light beams R and G is exposed in like manner to the blue light beam (B) that is transmitted through the shutter 40b. By repeating these processes of operation in the feeding direction of the sensitized sheet 60, an image of full-color display can be obtained.
In a direction perpendicular to the feeding direction of the sensitized sheet 60, an image is formed by means of the liquid crystal shutter 50.
Referring now to FIG. 17, there will be described exposure timing for the formation of an image by means of the conventional optical printer apparatus shown in FIG. 16.
In FIG. 17, it is supposed, for ease of illustration, that the sensitized sheet 60 is stationary and the optical head 10 moves in the direction of arrow Z. In order to indicate the color, R, G or B, of the light beam to which the sensitized sheet 60 is exposed, moreover, the sensitized sheet 60 is divided into three layers for convenience. Exposure of the sensitized sheet 60 to the light beam R is represented by the hatching on the first layer from the top, among the aforesaid three layers. Likewise, exposure to the light beam G and exposure to the light beam B are represented by hatching the second and third layers, respectively. It is, to be understood that FIG. 17 never illustrates the fact that the actual sensitized sheet 60 is composed of those three layers.
Sections {circle around (1)} to {circle around (6)} individually represent pixels in the moving direction (direction Z in FIG. 17) of the optical head. The width of each pixel is represented by X in FIG. 17.
Item (a) of FIG. 17 shows a state in which the light beam R starts to be radiated so that the optical head 10 exposes the section {circle around (3)} on the sensitized sheet 60 thereto. As this is done, the light beams G and B are not radiated. Then, the optical head 10 radiates the light beam R as it moves at uniform speed for the distance X (equal to the pixel width) in the direction of arrow Z. The exposure of the section {circle around (3)} to the light beam R terminates when the position of (b) of FIG. 17 is reached.
The moment the optical head 10 comes to the position of (b) of FIG. 17 to finish the radiation of the light beam R, the optical head 10 starts to radiate the light beam G for the section {circle around (3)}, as shown in (c) of FIG. 17. The section {circle around (3)} has already been exposed to the light beam R, as described above. Then, the optical head 10 radiates the light beam G as it moves at uniform speed for the distance X in the direction of arrow Z. The exposure of the section {circle around (3)} to the light beam G terminates when the optical head 10 comes to the position of (d) of FIG. 17.
The moment the optical head 10 comes to the position of (d) of FIG. 17 to finish the radiation of the light beam G, the optical head 10 starts to radiate the light beam B for the section {circle around (3)}, as shown in (e) of FIG. 17. The section {circle around (3)} has already been exposed to the light beams R and G, as described above. Then, the optical head 10 radiates the light beam B as it moves at uniform speed for the distance X in the direction of arrow Z. The exposure of the section {circle around (3)} to the light beam B terminates when the optical head 10 comes to the position of (f) of FIG. 17.
As described above, the section {circle around (3)} of the sensitized sheet 60 is exposed to the light beams R, G and B in a series of processes of operation shown in (a) to (f) of FIG. 17. This series of operation processes will hereinafter be referred to as an exposure cycle. In a second exposure cycle subsequent to this cycle, the section {circle around (6)} is exposed, as shown in (g) of FIG. 17.
In the conventional optical printer, as described above, a full-color image can be formed on the sensitized sheet 60 by continuously repeating the aforesaid exposure cycles.
According to the conventional optical printer apparatus arranged in this manner, however, the image pitch or spacing between images is equal to a maximum exposure distance (mentioned later), as mentioned before, so that the position of the section {circle around (3)}, which is situated at a distance 2X from the exposed section {circle around (3)}, is exposed between the first and second exposure cycles, as shown in (g) of FIG. 6.
Thus, according to the conventional optical printer apparatus, the image involves an unexposed portion (i.e., sections {circle around (4)} and {circle around (5)} that is twice as long as the exposure distance X between the exposure cycles, resulting in lowered resolution and image quality.
The object of the present invention is to provide an optical printer apparatus, capable of printing high-resolution, high-quality color images free from unexposed portions.
In order to achieve the above object, an optical printer apparatus according to the present invention comprises an optical head, capable of radiating a plurality of color light beams while moving relatively to a sensitized material, and a drive unit for driving the optical head and/or the sensitized material in order to cause the optical head and the sensitized material to move relatively to each other at constant speed, and is designed so that individual images formed on the sensitized material when the color light beams are radiated simultaneously are arranged at given pitches in the direction of the relative movement when the optical head is stationary with respect to the sensitized material, and that an image is formed on the sensitized material as the light beams are applied in regular order accompanying the relative movement of the optical head. Let P be the image pitch of the color light beams on the sensitized material and D be the maximum exposure distance corresponding to the maximum emission time of the color light beams for each pixel, the maximum exposure distance D is set smaller than the image pitch P.
According to the present invention, the whole area of the sensitized material can be exposed even in the case where the color light beams on the sensitized material cannot be focused in close vicinity to one another in the moving direction of the optical head, so that the resolution of the image can be improved. Since the gradation of a region between each two adjacent pixels is the average of the respective gradations of the pixels, a fine image with good color mixture can be obtained.